Modulation system



Aprila, 1934. T. NAKASHIMA Er AL 1,953,769

MODULATION SYSTEM Filed Sept. 3, 1952 Syvum/haii Tomomaso. NCLKCLSHIITIQ KQYU'LYO TKcLj @mail wat,

Patented Apr. 3, 1934 UNITED STATES PATENT OFFICE 1,953,769 Y MoDULATIoN SYSTEM Tomomasa Nakashima and Kenjiro Takayanag'l, Hamamatsu-Shi, Japan 4 Claims. (Cl. 179-171) Our invention relates to modulation systems and more particularly to modulation systems for televising moving views.

One object of our inventionl is'to provide a mod- 5 ulaton system for television which enables substantially uniform degree of modulation regardless of wide-range-variation of modulating current.

Another object of our invention is to provide the system specified using more than ten thousand view-elements with substantially uniform degree of modulation.

A further object of our invention is to obtain a. modulation system with substantially straightll line characteristic regardless of frequency variation of modulating current having an extremely wide frequency range from about ten cycles' per second to several hundred thousand cycles per second. l

A further object of our invention is to provide a television system capable of a clear reproduction of televised views.

A still further object of our invention is to provide a modulation system having a rising characteristic at the upper regiony of the frequency range of modulating current so as to improve appearance of received images.

There aref other objects and particularities of our invention, whichfwill be in part indicated in 180 the following description and in part rendered apparent therefrom inconnection with the accompanying drawing. l

In the system for televising moving pictures or other moving objectait isdesirable to use a large number of view-elements or picture-elements, say

ten thousand or above; inl order to secure a clear reproduction of televised views, and; if such a large number of view-elements are used, the modulating current variation takes place at a rate corre- 4.0 spending to a frequencywhich varies from about ten cycles to several hundred thousand cycles per second according to the rate of change of the darkness of successive view-elements swept by a scanning light-beam;

For such a wide frequency range of modulating current, modulation systemsheretofore been used in radio-telephony cannot operate satisfactorily at the upper and lower regions of the frequency range, because of the fact that, in usual platemodulation systems, an nductance or reactance coil is inserted in the plate circuit of a modulator tube in,` order to obtain modulating potential, and due to the distributed capacity of` the coil together with various lossestherein, it is practically impossible to` obtain an adequate modulating characteristic over such a wide frequency-range.

If the reactance coil is designed for satisfactory action at low frequencies about ten cycles per second, it cannot act adequately at high frequencies about several hundred thousand cycles per second, and vice versa.

In order to obviate such defects, we propose to use a pure resistance in place of the reactance coil. Resistance does not vary its value for frequency variation, and the modulating potential is directly proportional to current variation regardless of its frequency.

The modulation resistance inserted in the plate circuit of the modulator tube must be of such a value that the conductance of the circuit in parallel with the resistance, mainly due to the combined capacity presented by modulator tubes, wires and apparatus forming the plate circuit, andv carrier-wave oscillator tubes, etc. is s low that it does not substantially affect the conductance of the resistance circuit even at the upper region of the frequency range, in order to have an adequate and substantially denite proportion of the plate current of the modulator passing through the modulation resistance. 8@

This condition otherwise necessitates the use of an extremely low value of modulation resistance, and in order to obtain an adequate modulating potential, a great number of modulator tubes of large power must be used in parallel.

l From economical standpoint, the number of modulator tubes used in parallel must be as small as possible. For this purpose, the above-mentioned conductance of the circuit parallel to the modulation resistance must be made as low as possible, that is to say, the capacitance in parallel with the modulation resistance should be sufficiently small, even at the upper region of the frequency range.

In order to have the parallel static capacity small enough, we propose to use a so-called pushpull type carrier oscillation ampliier and a separate carrier wave oscillator as hereinafter fully described.

For a more complete understanding of the nature of the invention, reference should be had to the following description, taken in conjunction with the accompanying drawing, wherein:

Fig. l is a diagrammatic representation of a modulation system for television embodying our invention.

Fig. 2 is an equivalent circuit diagram of the system shown in Fig. l.

Referring to Fig. 1, a photoelectric cell 1 is disposed to receive the light ray 2 which is projected 110 thereon from successive view-elements of the views to be televised, not shown, and the output thereof is supplied to an amplier. The rectangle 3 represents the source of current to the cell 1 together wtih the amplifier of the output thereof, details thereof being not illustrated, because they do not constitute the essential part of our invention and may be of any construction wellknown in the art. rihe output potential from the ainpliiler 3 is applied to the control electrode or grid of a triode modulator tube 4.

A modulation resistance 5 is inserted in the plate circuit of the tube 4, and the modulating potential or the potential drop across the resistance 5 is applied to the plate electrode or anode of a push-pull type carrier oscillation amplifier generally shown by 6.

The carrier-wave oscillator 'E' is provided separately from the ampliiier 5, and the output thereof is supplied to the amplifier 6 through suitable coupling 8.

The amplifier 6 comprises a pair of shielded tetrode vacuum tubes 9, and a coupling coil 19 bridging the anode of both tubes 9 and having a neutral point 11 to which is connected one terminal of the modulation resistance 5.

The modulated radio frequency current through the plate circuit of the amplifier 6 is transformed to a radio frequency amplifier through a suitable coupling 13, and the output of the amplier l2 is radiated by means of well-known antenna system shown generally by 14.

Between the terminal of modulation resistance 5 and the neutral point 11 of the coil 10 is inserted a pure resistance 15 having a condenser 16 parallel-connected therewith, the function thereof being referred to hereinafter.

It will be understood that the radio frequency does not affect the plate of tube 4., because of the fact that the plate is connected to the neutral point 11 of the coupling coil 10 bridging the anodes of tubes 9, the two tubes 9 having identical characteristics; and further, because the potential of the point 11 is not aected by the plate current of both tubes 9. It is quite practical to make both tubes 9 identical in characteristics so that the potential of neutral point 11 is free from the variation of the plate current of tubes 9. We have succeeded in obtaining practically identical tubes 9 of considerable capacity for this purpose. For tubes 9 of extremely large capacity, however, there should be inserted a suitable radio-frequency choke coil between the point 11 and the tube 4, because identical characteristics are hardly obtained between the two tubes 9 of extremely large capacity.

As far the frequency range under question is concerned, an'equivalent circuit of the system illustrated may be drawn as shown in Fig. 2, in which represents the modulating electromotive force, 21 represents the internal resistance of the modulator tube 4. and 22 represents the static capacity of the plate electrode of the tube Il together with the wirings associated therewith. The internal resistance of the amplier tubes 9 is represented by 23, and the static capacity of the plate electrode thereof together with the wirings associated therewith, including the distributed capacity of the coil 10, is represented by 24.

As pointed out previously, the value of resistance 5 should be relatively low so that it permits an adequate flow of anode current therethrough which is not substantially aiected by the capacitances 22 and 24, the reactance of which varies with current frequency; in other words,

the combined conductance eifect of capacitances 22 and 24 should be sufficiently low even at the highest frequency of the modulating current.

According to our invention, by use of the separate oscillator 7 and the push-pull type amplifier 6, the capacitance 24 is so lowered that the above-mentioned condition is satisfactorily embodied without lowering the value of resistance 5 to impractically low value, because, in the pushpull type amplifier circuit, the anode supply can be connected to the neutral point 11 of the anode coil 19, and consequently no filtering condenser is necessary to be used, while when other type of self-exciting oscillator circuit or usual radio frequency amplifier is used as a modulated oscillator or amplifier, it is necessary to connect a by-pass condenser for the radio-frequency current to prevent the flow thereof through the anode supply. Such a by-pass condenser has a relatively large capacity.

According to our invention, the modulating potential can be made substantially proportional' to the anode current from the modulator tube 4, regardless of the frequency thereof, and the modulation is effected adequately throughout the whole range from about ten cycles to several hundred thousand cycles per second of the modulation current. For a practical purpose of increasing contrast of abruptly changing dar :ness of successive view-elements, it is desirable to give a rising characteristic to the modulation at the upper or high-frequency region of the frequency range, and for this purpose the resistance 15 and condenser 16 is disposed so as to provide a substantially pure and definite resistance to the flow of modulated current at the lower region of the frequency range, while at the upper region the combined reactance of the resistance 15 and capacitance 16 decreases and permits an exaggerated flow of the modulated current, thus giving a rising characteristic.

In practical use of our system for televising views, the modulation has somewhat lowering characteristic at the upper region of the frequency range, due to the static capacity of the photoelectric cell 1 together with wirings thereof, and due to the scanning operation in which the scanning light-beam has an appreciable crosssectional area, and the device 15-16 is preferably associated with the system in order to cornpensate for such undesirable characteristic.

In order to facilitate the understanding of our invention, a numerical example will be given below for a low power modulation system embodying our invention. When a 'UY- 224 tube is used for each tube 9, and four of ITX- 171A tube is used in multiple for the modulator 4., the capacity 22 is about 40 micro-microfarads, and the capacity 24 is about 30 micro-microfarads, and the resistance 5 may be about 2090 ohms. If the resistance 15 of about 2500 ohms and the condenser 16 of about 0.902 microfarad is used the modulation will have a rising characteristic at the upper region of the frequency range.

While we have described particular means for practising our invention, it will be recognized that this is only by way of illustration of general principles and that our invention is not limited to that particular means, but various changes, modifications and alterations may be made without departing from the spirit and scope of the invention as defined in the appended claims.

We claim as our invention:

1. In a modulation system, the combination with a three-electrode modulator tube, of a modulation resistance inserted in the anode circuit of said tube, a radio-frequency oscillator, a pushpull type radio-frequency amplifier associated with said oscillator and including a coil having a neutral point, and means for connecting one end of said resistance to said neutral point and for connecting said amplier in parallel circuit relation with said resistance.

2. In a modulation system,` the combination with a three-electrode modulator tube, of a modulation resistance inserted in the anode circuit of said tube, a radio-frequency oscillator, a pushpull type radio-frequency amplier associated With said oscillator and comprising a pair of amplier tubes each having an anode, and a coupling coil having a neutral point and bridging the anodes of said amplifier tubes, and means for connecting one end of said resistance to said neutral point and for connecting said amplier in parallel circuit relation with said resistance.

3. In a modulation system, the combination with a three-electrode modulator tube, of a modulation resistance inserted in the anode circuit of said tube, a radio-frequency oscillator, a pushpull type radio-frequency amplifier associated with said oscillator and comprising a pair of amplifier tubes each having an anode, and a. coupling coil having a neutral point and bridging the anodes of said amplier tubes, and means for connecting one end of said resistance to said neutral point and for connecting said amplier in parallel circuit relation with said resistance, said connecting means including a series resistance having a condenser parallel-connected therewith.

4. In a 4modulation system, the combination with a three-electrode modulator tube, of a modulation resistance inserted in the anode circuit of said tube, a radio-frequency oscillator, a pushpull type radio-frequency amplifier associated with said oscillator and comprising a pair of shielded four-electrode tubes, and a coupling coil having a neutral point and connecting the anodes of said four-electrode tubes together, and means for connecting one end of said resistance to said neutral point and for connecting said amplier in parallel relation with said resistance.

TOMOMASA NAKASHIMA. KENJIRO TAKAYANAGI. 

